Flexible silicon nanowire electrode

ABSTRACT

A method is presented for forming a nanowire electrode. The method includes forming a plurality of nanowires over a first substrate, depositing a conducting layer over the plurality of nanowires, forming solder bumps and electrical interconnections over a second flexible substrate, and integrating nanowire electrode arrays to the second flexible substrate. The plurality of nanowires are silicon (Si) nanowires, the Si nanowires used as probes to penetrate skin of a subject to achieve electrical biopotential signals. The plurality of nanowires are formed over the first substrate by metal-assisted chemical etching.

BACKGROUND Technical Field

The present invention relates generally to nanowire devices, and morespecifically, to a flexible silicon nanowire electrode.

Description of the Related Art

Measuring biopotential signals, such as electrocardiogram (ECG),electroencephalogram (EEG), electrooculogram (EOG), and electromyogram(EMG) signals, is significant for both research and clinicalapplications. Changes in these electrical biopotential signals can beused to record heart rate, assess muscle contraction, mechanics orinferring movement rates across the muscle, study brain activity, etc.All these measurements can be very useful in basic physiology,behavioral or pharmacological/toxicology studies.

Electrodes are key components for continuous physiological monitoring.Conventional wet electrodes, which usually require skin preparation andthe use of conductive gels to reduce skin-sensor interface impedance,can be uncomfortable for many users.

SUMMARY

In accordance with an embodiment, a method is presented for forming ananowire electrode. The method includes forming a plurality of nanowiresover a first substrate, depositing a conducting layer over the pluralityof nanowires, forming electrical interconnections and solder bumps overthe second flexible substrate, and integrating nanowire electrode arraysto the flexible substrate. The plurality of nanowires are silicon (Si)nanowires, the Si nanowires used as probes to penetrate skin of asubject to achieve electrical biopotential signals. The plurality ofnanowires are formed over the first substrate by metal-assisted chemicaletching, and bio-compatible conductive coating and high-doped substrateare used to improve mechanical strength of electrodes and achievelow-impedance electrodes, respectively.

In accordance with an embodiment, a flexible silicon nanowire electrodestructure is provided. The nanowire electrode structure includes aplurality of silicon (Si) nanowires formed by metal-assisted chemicaletching on a first substrate, the plurality of Si nanowires coated witha conducting layer and a plurality of nanowire chips integrated within asecond substrate, the second substrate being a flexible substratecontacting solder bumps formed adjacent the first substrate. Theplurality of Si nanowires are used as probes to penetrate skin of asubject.

It should be noted that the exemplary embodiments are described withreference to different subject-matters. In particular, some embodimentsare described with reference to method type claims whereas otherembodiments have been described with reference to apparatus type claims.However, a person skilled in the art will gather from the above and thefollowing description that, unless otherwise notified, in addition toany combination of features belonging to one type of subject-matter,also any combination between features relating to differentsubject-matters, in particular, between features of the method typeclaims, and features of the apparatus type claims, is considered as tobe described within this document.

These and other features and advantages will become apparent from thefollowing detailed description of illustrative embodiments thereof,which is to be read in connection with the accompanying drawings.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

The invention will provide details in the following description ofpreferred embodiments with reference to the following figures wherein:

FIG. 1 is a cross-sectional view of a semiconductor structure having asubstrate attached to a glass handler, in accordance with an embodimentof the present invention;

FIG. 2 is a cross-sectional view of the semiconductor structure of FIG.1 where the substrate is thinned, in accordance with an embodiment ofthe present invention;

FIG. 3 is a cross-sectional view of the semiconductor structure of FIG.2 where metal-assisted chemical etching takes place to form a pluralityof silicon (Si) nanowires on a Si wafer, in accordance with anembodiment of the present invention;

FIG. 4 is a cross-sectional view of the semiconductor structure of FIG.3 where a conducting layer is deposited over the plurality of Sinanowires, in accordance with an embodiment of the present invention;

FIG. 5 is a cross-sectional view of the semiconductor structure of FIG.4 where the glass handler is removed, in accordance with an embodimentof the present invention;

FIG. 6 is a cross-sectional view of another semiconductor structurewhere a flexible polymer substrate is attached to another glass handler,in accordance with an embodiment of the present invention;

FIG. 7 is a cross-sectional view of the semiconductor structure of FIG.6 where interconnection lines are formed within the flexible substrateand solder bumps are deposited adjacent the interconnection lines, thesolder bumps used to join nanowire chips to the flexible substrate, inaccordance with an embodiment of the present invention;

FIG. 8 is a cross-sectional view of the semiconductor structure of FIG.7 where the electrode arrays of FIG. 4 are integrated with the solderbumps, in accordance with an embodiment of the present invention;

FIG. 9 is a cross-sectional view of the semiconductor structure of FIG.8 where the glass handler is released, in accordance with an embodimentof the present invention;

FIG. 10 is a top view of the semiconductor structure of FIG. 9, inaccordance with an embodiment of the present invention;

FIG. 11 is a perspective view of the flexible silicon nanowire electrodeof FIG. 9 in contact with skin of a subject, in accordance with anembodiment of the present invention; and

FIG. 12 is a block/flow diagram of an exemplary method for forming aflexible silicon nanowire electrode, in accordance with an embodiment ofthe present invention.

Throughout the drawings, same or similar reference numerals representthe same or similar elements.

DETAILED DESCRIPTION

In one or more embodiments, a method is presented for forming a nanowireelectrode. The method includes forming a plurality of nanowires over afirst substrate, depositing a conducting layer over the plurality ofnanowires, forming solder bumps adjacent the first substrate, the solderbumps in opposed relation to the plurality of nanowires, and forming asecond substrate adjacent the solder bumps, the second substrateincluding interconnection lines in electrical communication with thesolder bumps. The plurality of nanowires are silicon (Si) nanowires, theSi nanowires used as probes to penetrate skin of a subject. Theplurality of nanowires are formed over the first substrate bymetal-assisted chemical etching.

In one or more embodiments, a nanowire electrode is provided. Thenanowire electrode includes a plurality of nanowires formed over a firstsubstrate, a conducting layer deposited over the plurality of nanowires,solder bumps formed adjacent the first substrate and in opposed relationto the plurality of nanowires, and a second substrate formed adjacentthe solder bumps, the second substrate including interconnection linesin electrical communication with the solder bumps.

In one or more embodiments, a nanowire electrode structure is provided.The nanowire electrode structure includes a plurality of silicon (Si)nanowires formed by metal-assisted chemical etching on a firstsubstrate, the plurality of Si nanowires coated with a conducting layerand a plurality of nanowire chips integrated within a second substrate,the second substrate being a flexible substrate contacting solder bumpsformed adjacent the first substrate. The plurality of Si nanowires areused as probes to penetrate skin of a subject. The plurality of Sinanowires penetrate at least a stratum basale layer (or StratumGerminativum (SG) layer) of the skin of the subject.

In one or more embodiments, silicon nanowires are used to penetrate theelectrically conducting Stratum Germinativum (SG) layer of the skin tocircumvent the high impedance at the skin-sensor interface. The assemblyincludes solid electrode arrays formed on a flexible substrate toconstruct large-area flexible nanowire electrodes. The methods use ahigh-doped silicon substrate and metal over-coating to construct lowimpedance nanowire electrodes. The methods further use a bio-compatiblemetal over-coating on the formed silicon nanowires to improve themechanical strength of the nanowire electrodes. The methods further usea low-temperature assembly approach to integrate multi-layers andstructures.

In one or more embodiments, silicon nanowires are formed bymetal-assisted chemical etching on a silicon wafer. These nanowires areused to penetrate at least the electrically conducting StratumGerminativum (SG) layer of the skin to circumvent the high impedance atthe skin-sensor interface. By using an array of nanowires on a flexiblesubstrate, a flexible silicon nanowire electrode for at least clinicalapplications can be achieved. The nanowire electrode techniquesdescribed herein can be used to measure, e.g., at leastelectroencephalogram (EEG) signals, electrocardiogram (ECG) signals,electrooculography (EOG) signals, and/or electromyography (EMG) signals.

In one or more embodiments, the silicon nanowires have a length of about10-100 um and a diameter of about 10 nm to 1 um for use aselectrodes/probes. The silicon nanowires are formed over a thin siliconsubstrate on top of a flexible polymer substrate to achieve flexibleelectrode patches. Additionally, the silicon nanowires include abio-compatible metal over-coating to prevent, e.g., skin irritation.

In one or more embodiments, highly-doped silicon nanowires areintegrated with a metal over-coating to construct low resistancenanowire electrodes/probes. Additionally, solid silicon nanowire chipsare integrated on a flexible polymer substrate to make flexibleelectrodes for clinical application. The bio-compatible low-temperatureassembly methods use low-temperature wafer bonding, glass handlerreleasing, wafer transfer, and low-temperature solder bump bonding toachieve multi-layer integration.

In one or more embodiments, silicon nanowires are used as a probe.Fabrication of the silicon nanowires is accomplished by a wet etchingmethod referred to as metal-assisted chemical etching, which results ina low-temperature assembly approach to integrate nanowire arrays on aflexible substrate. The silicon nanowires are of a sub-micro width to beused as probes to penetrate human skin. In one or more embodiments, thesilicon nanowires are directly etched on a silicon semiconductor layerand the nanowire structure is built by using a conductive siliconsubstrate, metal coating over the nanowires, and backside metal bump toachieve electrical connections to nanowire chips formed within orembedded within a flexible substrate. The silicon nanowires act as apenetrating probe to penetrate the skin of a subject for more accurateand sensitive signal measurement.

As used herein, “semiconductor device” refers to an intrinsicsemiconductor material that has been doped, that is, into which a dopingagent has been introduced, giving it different electrical propertiesthan the intrinsic semiconductor. Doping involves adding dopant atoms toan intrinsic semiconductor, which changes the electron and hole carrierconcentrations of the intrinsic semiconductor at thermal equilibrium.Dominant carrier concentration in an extrinsic semiconductor determinesthe conductivity type of the semiconductor.

As used herein, the term “drain” means a doped region in thesemiconductor device located at the end of the channel, in whichcarriers are flowing out of the transistor through the drain.

As used herein, the term “source” is a doped region in the semiconductordevice, in which majority carriers are flowing into the channel.

The term “direct contact” or “directly on” means that a first element,such as a first structure, and a second element, such as a secondstructure, are connected without any intermediary conducting, insulatingor semiconductor layers at the interface of the two elements.

The terms “overlying”, “atop”, “positioned on” or “positioned atop”means that a first element, such as a first structure, is present on asecond element, such as a second structure, wherein interveningelements, such as an interface structure can be present between thefirst element and the second element.

The term “electrically connected” means either directly electricallyconnected, or indirectly electrically connected, such that interveningelements are present; in an indirect electrical connection, theintervening elements can include inductors and/or transformers.

The term “crystalline material” means any material that issingle-crystalline, multi-crystalline, or polycrystalline.

The term “non-crystalline material” means any material that is notcrystalline; including any material that is amorphous, nano-crystalline,or micro-crystalline.

The term “intrinsic material” means a semiconductor material which issubstantially free of doping atoms, or in which the concentration ofdopant atoms is less than 10¹⁵ atoms/cm³.

As used herein, “p-type” refers to the addition of impurities to anintrinsic semiconductor that creates deficiencies of valence electrons.In a silicon-containing substrate, examples of n-type dopants, i.e.,impurities, include but are not limited to: boron, aluminum, gallium andindium.

As used herein, “n-type” refers to the addition of impurities thatcontributes free electrons to an intrinsic semiconductor. In a siliconcontaining substrate examples of n-type dopants, i.e., impurities,include but are not limited to antimony, arsenic and phosphorous.

As used herein, an “anisotropic etch process” denotes a material removalprocess in which the etch rate in the direction normal to the surface tobe etched is greater than in the direction parallel to the surface to beetched. The anisotropic etch can include reactive-ion etching (RIE).Other examples of anisotropic etching that can be used include ion beametching, plasma etching or laser ablation.

The term “processing” as used herein includes deposition of material orphotoresist, patterning, exposure, development, etching, cleaning,stripping, implanting, doping, stressing, layering, and/or removal ofthe material or photoresist as required in forming a describedstructure.

As used herein, “depositing” can include any now known or laterdeveloped techniques appropriate for the material to be depositedincluding but not limited to, for example: chemical vapor deposition(CVD), low-pressure CVD (LPCVD), plasma-enhanced CVD (PECVD),semi-atmosphere CVD (SACVD) and high density plasma CVD (HDPCVD), rapidthermal CVD (RTCVD), ultra-high vacuum CVD (UHVCVD), limited reactionprocessing CVD (LRPCVD), metal-organic CVD (MOCVD), sputteringdeposition, ion beam deposition, electron beam deposition, laserassisted deposition, thermal oxidation, thermal nitridation, spin-onmethods, physical vapor deposition (PVD), atomic layer deposition (ALD),chemical oxidation, molecular beam epitaxy (MBE), plating, evaporation.

The word “exemplary” is used herein to mean “serving as an example,instance, or illustration.” Any embodiment described herein as“exemplary” is not necessarily to be construed as preferred oradvantageous over other embodiments. Likewise, the term “embodiments ofthe invention” does not require that all embodiments of the inventioninclude the discussed feature, advantage or mode of operation.

It is to be understood that the present invention will be described interms of a given illustrative architecture; however, otherarchitectures, structures, substrate materials and process features andsteps/blocks can be varied within the scope of the present invention. Itshould be noted that certain features cannot be shown in all figures forthe sake of clarity. This is not intended to be interpreted as alimitation of any particular embodiment, or illustration, or scope ofthe claims.

Various illustrative embodiments of the invention are described below.In the interest of clarity, not all features of an actual implementationare described in this specification. It will of course be appreciatedthat in the development of any such actual embodiment, numerousimplementation-specific decisions must be made to achieve thedevelopers' specific goals, such as compliance with system-related andbusiness-related constraints, which will vary from one implementation toanother. Moreover, it will be appreciated that such a development effortmight be complex and time-consuming, but would nevertheless be a routineundertaking for those of ordinary skill in the art having the benefit ofthis invention.

The present subject matter will now be described with reference to theattached figures. Various structures, systems and devices areschematically depicted in the drawings for purposes of explanation onlyand so as to not obscure the present invention with details that arewell known to those skilled in the art. Nevertheless, the attacheddrawings are included to describe and explain illustrative examples ofthe present invention. The words and phrases used herein should beunderstood and interpreted to have a meaning consistent with theunderstanding of those words and phrases by those skilled in therelevant art. No special definition of a term or phrase, i.e., adefinition that is different from the ordinary and customary meaning asunderstood by those skilled in the art, is intended to be implied byconsistent usage of the term or phrase herein. To the extent that a termor phrase is intended to have a special meaning, i.e., a meaning otherthan that understood by skilled artisans, such a special definition willbe expressly set forth in the specification in a definitional mannerthat directly and unequivocally provides the special definition for theterm or phrase.

In the art of nanotechnology, nanowires are usually interpreted asnanostructures having a lateral size (e.g., diameter for cylindricalnanowires or width for pyramidal or hexagonal nanowires) of nano-scaleor nanometer dimensions, whereas its longitudinal size is unconstrained.Such nanostructures are commonly also referred to as nanowhiskers,one-dimensional nano-elements, nanorods, nanotubes, etc. Generally,nanowires with a polygonal cross section are considered to have at leasttwo dimensions each of which are not greater than 300 nm. However, thenanowires can have a diameter or width of up to about 1 μm. The onedimensional nature of the nanowires provides unique physical, opticaland electronic properties. These properties can for example be used toform devices utilizing quantum mechanical effects (e.g., using quantumwires) or to form heterostructures of compositionally differentmaterials that usually cannot be combined due to large lattice mismatch.As the term “nanowire” implies, the one dimensional nature is oftenassociated with an elongated shape. In other words, “one dimensional”refers to a width or diameter less than 1 micron and a length greaterthan 1 micron. Since nanowires can have various cross-sectional shapes,the diameter is intended to refer to the effective diameter. Byeffective diameter, it is meant the average of the major and minor axisof the cross-section of the structure.

A nanowire, as the semiconducting region, can be manufactured by avariety of processes. Depending on the actual geometry of wrapped carbonsheets forming a nanotube, the electric properties can be metallic,insulating or semiconducting. Hence, a nanotube can serve as a nanowirein certain embodiments of the present invention. Further, nanowires canbe produced by lithography on a semiconductor substrate or otherconventional methods. The cross-sectional geometry of the nanowire, inprinciple, is irrelevant. Therefore, one can fabricate nanowires havingregular cross-section as well as wires having irregular geometries.Further, a corresponding semiconductor nanowire can be engineered to bean n-type or a p-type semiconductor.

Nanowire semiconductor devices include a nanowire formed from asemiconductor material. The channel region of the nanowire is oftensuspended above a substrate such that the gate materials surround thechannel region of the nanowire. This forms a gate-all-around devicewhere the gate is arranged 360 degrees about the channel region.Nanowires can be stacked vertically to increase the density of the fieldeffect transistor (FET) devices on the substrate. Metal oxidesemiconductor field effect transistor (MOSFET) devices such as p-type(pFET) and n-type (nFET) devices are often formed from differentsemiconductor materials. For example, nFET devices having a siliconchannel region and pFET devices having silicon germanium channel regionsoffer performance advantages in complementary metal oxide semiconductor(CMOS) devices.

Nanowires made of silicon (Si) are especially attractive because ofsilicon's compatibility with existing integrated circuit (IC) processes.Moreover, the chemical and physical properties of silicon can becontrolled to adjust the device sensitivity, and silicon nanowires canbe selectively grown. Using silicon allows the vast knowledge of silicontechnology to be applied to applications such as sensing. Usingsemiconductor nanowires, researchers have demonstrated electricalsensors for biological and chemical species, and designed a range ofnano-electronic and photonic devices in different material systems. Inmany of these demonstrations, nanowires were assembled after growth intoparallel or crossed arrays by alignment aided by fluid flow or byapplying electric fields. In other cases, electrical contacts weredefined with electron-beam lithography on a few selected nanowires.

Silicon (Si) nanowires, with unique physio-chemical properties, havebrought significant breakthroughs in fields such as electronic devices,biochemical sensors, thermoelectric devices, solar cells, andelectrochemical energy conversion and storage devices. In some cases,single crystalline silicon nanowires are preferred over polycrystalline(poly-Si) and amorphous silicon (a-Si) nanowires for use in theapplications of electronic devices, biochemical sensors andthermoelectric devices, because they can have fewer defects and can bestronger and more conductive than polycrystalline and amorphous siliconnanowires of similar diameter.

FIG. 1 is a cross-sectional view of a semiconductor structure having asubstrate attached to a glass handler, in accordance with an embodimentof the present invention.

A semiconductor structure 5 includes a semiconductor substrate 10attached to a glass handler 20 via an adhesive bonding layer 12.

In one example embodiment, high-doped silicon (e.g., 0.001-0.005 Ohm-cmresistance) can be used to form substrate 10 for low impedanceelectrodes. The bonding layer 12 can be applied by a low temperaturebonding method, such as adhesive bonding, which can be used to join thesubstrate 10 to the glass handler 20. A glass wafer can be used as thehandler 20 for process compatibility.

In one or more embodiments, the substrate 10 can be a semiconductor oran insulator with an active surface semiconductor layer. The substrate10 can be crystalline, semi-crystalline, microcrystalline, or amorphous.The substrate 10 can be essentially (i.e., except for contaminants) asingle element (e.g., silicon), primarily (i.e., with doping) of asingle element, for example, silicon (Si) or germanium (Ge), or thesubstrate 10 can include a compound, for example, Al₂O₃, SiO₂, GaAs,SiC, or SiGe. The substrate 10 can also have multiple material layers,for example, a semiconductor-on-insulator substrate (SeOI), asilicon-on-insulator substrate (SOI), germanium-on-insulator substrate(GeOI), or silicon-germanium-on-insulator substrate (SGOI). Thesubstrate 10 can also have other layers forming the substrate 10,including high-k oxides and/or nitrides. In one or more embodiments, thesubstrate 10 can be a silicon wafer. In an embodiment, the substrate 10is a single crystal silicon wafer.

FIG. 2 is a cross-sectional view of the semiconductor structure of FIG.1 where the substrate is thinned, in accordance with an embodiment ofthe present invention.

In various embodiments, the substrate 10 can be thinned to a desiredthickness to form substrate 10′. The desired thickness can be about10-100 μm, which is equivalent to the thickness of the stratum corneumlayer of skin. In one example, mechanical grinding andchemical-mechanical polishing can be used to thin or etch the substrate10 to form substrate 10′.

In one embodiment, the etch process can be an anisotropic etch process,such as reactive ion etch (RIE). The etching can include a dry etchingprocess such as, for example, reactive ion etching, plasma etching, ionetching or laser ablation. The etching can further include a wetchemical etching process in which one or more chemical etchants are usedto remove portions of the blanket layers that are not protected by thepatterned photoresist. The patterned photoresist can be removedutilizing an ashing process.

In various embodiments, the height of the substrate can be selectivelyreduced or thinned by chemical-mechanical polishing (CMP) and/oretching. Therefore, the planarization process can be provided by CMP.Other planarization process can include grinding and polishing.

As used herein, the term “selective” in reference to a material removalprocess denotes that the rate of material removal for a first materialis greater than the rate of removal for at least another material of thestructure to which the material removal process is being applied. Forexample, in one embodiment, a selective etch can include an etchchemistry that removes a first material selectively to a second materialby a ratio of 10:1 or greater, e.g., 100:1 or greater, or 1000:1 orgreater.

FIG. 3 is a cross-sectional view of the semiconductor structure of FIG.2 where metal-assisted chemical etching takes place to form a pluralityof silicon (Si) nanowires on a Si wafer, in accordance with anembodiment of the present invention.

In various embodiments, silicon nanowires 16 are formed over theremaining substrate 14 by dipping the substrate 10′ into chemicals forseveral minutes to perform metal-assisted chemical etching.

Metal-assisted chemical etching offers a method of etching silicon bypatterning a silicon surface with a layer of metal. The metal acts as acatalyst for etching the silicon surface directly beneath it whenexposed to an oxidizing agent (e.g., H₂O₂) and an acid (e.g., HF).Metal-assisted chemical etching can produce nanowires of high aspectratios in silicon. Since it is a wet etch, metal-assisted chemicaletching can easily be done in large quantities for a low price comparedto popular dry etch methods that can require a vacuum or plasma. Also,the ability to make structures that are undamaged in any shape capableof being patterned with metal (e.g., gold) has made metal-assistedchemical etching a viable method of silicon nanowire fabrication.

Metal-assisted chemical etching is an electroless chemical etchingtechnique that can etch micron (e.g., about 1-1000 μm), sub-micron(e.g., about 0.1-1 μm), and nano-sized (e.g., 0.5-100 nm) features insubstrates or wafers with high aspect ratios. Metal-assisted chemicaletching uses a non-spherical metal catalyst deposited onto a substrateto locally increase the dissolution rate of the substrate material in anetchant solution including a fluoride etchant such as hydrofluoric acid(HF) and an oxidizing agent such as hydrogen peroxide (H₂O₂).

Substrate material including silicon (Si) is preferred. However,substrate materials can include germanium (Ge), arsenic (As), selenium(Se), gallium (Ga), tellurium (Te), polonium (Po), and combinationsthereof, as well as mixtures of group III, IV, and V compounds such asGa—As, Si—C, Ga—N, Si—N, Ga—Si, and Si—As. Substrate materials can alsobe doped with a dopant such as, but not limited to, boron (B),phosphorous (P), arsenic (As), gallium (Ga). Metal catalysts caninclude, but are not limited to, gold (Au), silver (Ag), platinum (Pt),tungsten (W), palladium (Pd), copper (Cu), and combinations and/oralloys thereof. Other metal catalysts can also include aluminum (Al),titanium (Ti), Nickel (Ni), iron (Fe), zinc (Zn), cadmium (Cd), indium(In), tin (Sn), antimony (Sb), tellurium (Te), lead (Pb), bismuth (Bi),vanadium (V), chromium (Cr), manganese (Mg), ruthenium (Ru), molybdenum(Mo), and other transition metals. Fluoride etchants also include, butare not limited to, buffered oxide etch (BOE), boron hydrogen fluoride(BHF), or other fluoride complex (e.g., BF₄—, PF₆—, CF₃SO₃—, AsF₆—, andSbF₆—). Other oxidizing agents that can be used include, but are notlimited to, K₂MnO₄ and FeNO₃.

In exemplary embodiments, the growth density of the nanowires 16 on thegrowth surface is from one to a few nanowires (e.g., 1 to 10, 1 to 3, upto 15). In other applications, many nanowires 16 may be desirable (e.g.,up to 50 or up to 75). The total conductance between the originatingsurface on an electrode and the contacted surface on another electrodedepends on density of nanoparticles, diameter of nanowires and the sizeof electrodes from which the wires are growing. A higher density ofnanoparticles on the growth surface results in a higher density ofnanowires, which generally results in a higher total conductance.

Nanoparticle sizes, and thus the resulting radial diameters, are in therange of less than 100 nm, alternatively 1 to 5 nm, and alternatively 5to 50 nm, and further alternatively less than 40 nm. In exemplaryembodiments, the diameter of the nanowire 16 is sufficient to havediffusion lengths and diffusion kinetics of the dopant species to allowdesired switching times at a programming temperature, e.g., greater thanor equal to about 700° C. to 800° C., preferably at or above 800° C. upto or above 1000° C., which is much greater than the operatingtemperature, e.g., from room temperature to about 100° C.

The programming temperature is at least partially dependent on thesemiconductor material of the nanowire. For example, a programmingtemperature for Si is greater than about 800° C. Also, a nanowiredimension can be configured small enough for rapid diffusion and shortdiffusion times at a programming temperature and slow diffusion and longdiffusion times at operating temperatures. It is noted that programmingtime is proportional to diffusion length (L), where L=2√{square rootover (Dt)}. D is the diffusion coefficient, and t is the time atelevated temperature during which the dopant atoms can move within thenanowire. In exemplary embodiments, the diffusion length (L) is lessthan about 1 μm, alternatively less than 100 nm, 50 nm, 40 nm or 10 nm.The diameter can be selected to be compatible with a desired programmingspeed. It is noted that faster speed can result from a smaller diameteras well as result from programming at higher programming temperature, ora combination of these approaches.

In various embodiments, the materials and layers can be deposited byphysical vapor deposition (PVD), chemical vapor deposition (CVD), atomiclayer deposition (ALD), molecular beam epitaxy (MBE), or any of thevarious modifications thereof, for example plasma-enhanced chemicalvapor deposition (PECVD), metal-organic chemical vapor deposition(MOCVD), low pressure chemical vapor deposition (LPCVD), electron-beamphysical vapor deposition (EB-PVD), and plasma-enhanced atomic layerdeposition (PE-ALD). The depositions can be epitaxial processes, and thedeposited material can be crystalline. In various embodiments, formationof a layer can be by one or more deposition processes, where, forexample, a conformal layer can be formed by a first process (e.g., ALD,PE-ALD, etc.) and a fill can be formed by a second process (e.g., CVD,electrodeposition, PVD, etc.).

FIG. 4 is a cross-sectional view of the semiconductor structure of FIG.3 where a conducting layer is deposited over the plurality of Sinanowires, in accordance with an embodiment of the present invention.

In various embodiments, a metal layer 19 is deposited and the substrate14 is broken into a plurality of substrates 18, each substrate 18configured to accommodate a plurality of nanowires 16 coated with metallayer 19. The metal layer coating 19 enhances biocompatibility andimproves the mechanical strength of the nanowires 16. Metal depositionmethods can include, e.g., PVD, plating, evaporation, etc. The metallayer 19 can be, e.g., a biocompatible metal, such as titanium nitride(TiN).

FIG. 5 is a cross-sectional view of the semiconductor structure of FIG.4 where the glass handler is removed, in accordance with an embodimentof the present invention.

In various embodiments, the glass handler 20 is removed, thus leavingbehind a plurality of nanowire electrodes 25 placed over the bondinglayer 12.

FIG. 6 is a cross-sectional view of another semiconductor structurewhere a flexible polymer substrate is attached to another glass handler,in accordance with an embodiment of the present invention.

A semiconductor structure 7 includes a polymer layer 34 attached to aglass handler 30 via a release layer 32. The release layer assists theglass handler releasing process, and the polymer layer 34 can be used asa flexible substrate of the final nanowire electrode, as describedfurther below.

FIG. 7 is a cross-sectional view of the semiconductor structure of FIG.6 where interconnection lines are formed within the flexible substrateand solder bumps are deposited adjacent the interconnection lines, thesolder bumps used to join nanowire chips to the flexible substrate, inaccordance with an embodiment of the present invention.

In various embodiments, conducting lines 36 are fabricated within apolymer layer 37 deposited over the flexible polymer layer 34.Additionally, solder bumps 38 are deposited over the conducting lines36. The solder bumps 38 do not contact each other. The solder bumps 38are separated from each other by a predetermined distance. Theconducting lines 36 can be metal lines. The metal lines 36 can be usedas electrical interconnections between nanowire chips. The solder bumps38 can be used to join the nanowire chips to the flexible polymersubstrate 34. The solder bumps 38 can also be referred to as soldernanodots or solder films or solder ball bumps.

FIG. 8 is a cross-sectional view of the semiconductor structure of FIG.7 where the electrode arrays of FIG. 4 are integrated with the solderbumps, in accordance with an embodiment of the present invention.

In various embodiments, the electrodes 25 of FIG. 5 are attached to thesolder bumps 38 such that each electrode array 25 is associated with asingle solder bump 38. The electrodes 25 are attached such thatrespective solder bumps 38 contact the respective substrate 18 of eachof the electrode arrays 25. The solder bumps 38 separate the metal lines36 from the electrode arrays 25.

FIG. 9 is a cross-sectional view of the semiconductor structure of FIG.8 where the glass handler is released, in accordance with an embodimentof the present invention.

In various embodiments, the glass handler 30 and the release layer 32are removed, thus leaving behind a plurality of nanowire electrodes 25placed over the flexible substrate 34. The nanowire structure 40 isready to be applied to the skin of a subject, e.g., a patient. Thesilicon nanowires coated with a metal layer are configured to penetratethe skin of a patient. Specifically, the silicon nanowires are designedto penetrate at least the stratum germinativum (SG) layer (or stratumbasale layer) of the skin, as described further below with reference toFIG. 11.

FIG. 10 is a top view of the semiconductor structure of FIG. 9, inaccordance with an embodiment of the present invention.

The top view illustrates each electrode array. The top coated surface 19of the nanowires 16 is showed surrounded by substrates 18. Metal lines36 are also shown connecting the electrode arrays 25 to each other.

FIG. 11 is a perspective view of the flexible silicon nanowire electrodeof FIG. 9 in contact with skin of a subject, in accordance with anembodiment of the present invention.

In various embodiments, the nanowire structure 40 is used to penetratethe skin 50 of a subject. The skin 50 is composed of three basic layersor regions. The first layer 52 is the epidermis. The second layer 54 isthe dermis. The third layer 56 is the hypodermis. The first layer 52includes 5 different layers. These layers are the stratum corneum layer,the stratum lucidum layer, the stratum granulosum layer, the stratumspinosum layer, and the stratum basale layer (also referred to as thestratum germinativum layer). The stratum basale layer contacts thedermis region 54. The dermis region can include two different layers,that is, the papillary region and the reticular region. Therefore, thestratum basale layer is adjacent the dermis region 54 or the farthestaway from the outer surface of the skin of the subject, yet within theepidermis region 52.

The nanowires 16 coated with the metal layer 19 are configured topenetrate the skin 50 such that the nanowires 16 reach at least thestratum germinativum layer of the epidermis 52 adjacent the dermis 54.Thus, the nanowires 16 extend to a distalmost region of the epidermis 52(i.e., the stratum basale layer or stratum germinativum layer). As aresult, the silicon nanowires penetrate the electrically conductivestratum germinativum (SG) layer to circumvent the high impedance at theskin sensor interface. The stratum germinativum (SG) layer can beapproximately 50-100 μm deep.

FIG. 12 is a block/flow diagram of an exemplary method for forming aflexible silicon nanowire electrode, in accordance with an embodiment ofthe present invention.

At block 62, a plurality of nanowires are formed over a first substrate.

At block 64, a conducting layer is deposited over the plurality ofnanowires to achieve an electrode array.

At block 66, solder bumps and interconnection lines are formed over asecond flexible substrate.

At block 68, the nanowire electrode array is integrated with the secondflexible substrate.

The nanostructures of the exemplary embodiments have been illustrativelyshown to be used for medical device applications. However, thenanostructures of the exemplary embodiments of the present invention canbe used in many applications including, but not limited to, solar cells,light emitting diodes (LEDs), laser diodes (LDs) such as distributedfeedback (DFB) lasers and distributed Bragg reflector (DBR) lasers,detectors, field effect transistors (FETs), thermoelectric devices,sensors such as along biological interfaces, and nanoscale processingunits.

Methods as described herein can be used in the fabrication of integratedcircuit chips and/or solar cells. The resulting integrated circuit chipsor cells can be distributed by the fabricator in raw wafer form (thatis, as a single wafer that has multiple unpackaged chips), as a baredie, or in a packaged form. In the latter case, the chip is mounted in asingle chip package (such as a plastic carrier, with leads that areaffixed to a motherboard or other higher level carrier) or in amultichip package (such as a ceramic carrier that has either or bothsurface interconnections or buried interconnections). In any case, thechip is then integrated with other chips, discrete circuit elements,and/or other signal processing devices as part of either (a) anintermediate product, such as a motherboard, or (b) an end product. Theend product can be any product that includes photovoltaic devices,integrated circuit chips with solar cells, ranging from toys,calculators, solar collectors and other low-end applications to advancedproducts for medical applications, as described herein.

It is to be understood that the present invention will be described interms of a given illustrative architecture; however, otherarchitectures, structures, substrate materials and process features andsteps/blocks can be varied within the scope of the present invention.

It will also be understood that when an element such as a layer, regionor substrate is referred to as being “on” or “over” another element, itcan be directly on the other element or intervening elements can also bepresent. In contrast, when an element is referred to as being “directlyon” or “directly over” another element, there are no interveningelements present. It will also be understood that when an element isreferred to as being “connected” or “coupled” to another element, it canbe directly connected or coupled to the other element or interveningelements can be present. In contrast, when an element is referred to asbeing “directly connected” or “directly coupled” to another element,there are no intervening elements present.

The present embodiments can include a design for an integrated circuitchip, which can be created in a graphical computer programming language,and stored in a computer storage medium (such as a disk, tape, physicalhard drive, or virtual hard drive such as in a storage access network).If the designer does not fabricate chips or the photolithographic masksused to fabricate chips, the designer can transmit the resulting designby physical mechanisms (e.g., by providing a copy of the storage mediumstoring the design) or electronically (e.g., through the Internet) tosuch entities, directly or indirectly. The stored design is thenconverted into the appropriate format (e.g., GDSII) for the fabricationof photolithographic masks, which typically include multiple copies ofthe chip design in question that are to be formed on a wafer. Thephotolithographic masks are utilized to define areas of the wafer(and/or the layers thereon) to be etched or otherwise processed.

Methods as described herein can be used in the fabrication of integratedcircuit chips. The resulting integrated circuit chips can be distributedby the fabricator in raw wafer form (that is, as a single wafer that hasmultiple unpackaged chips), as a bare die, or in a packaged form. In thelatter case, the chip is mounted in a single chip package (such as aplastic carrier, with leads that are affixed to a motherboard or otherhigher level carrier) or in a multichip package (such as a ceramiccarrier that has either or both surface interconnections or buriedinterconnections). In any case, the chip is then integrated with otherchips, discrete circuit elements, and/or other signal processing devicesas part of either (a) an intermediate product, such as a motherboard, or(b) an end product. The end product can be any product that includesintegrated circuit chips, ranging from toys and other low-endapplications to advanced computer products having a display, a keyboardor other input device, and a central processor.

It should also be understood that material compounds will be describedin terms of listed elements, e.g., SiGe. These compounds includedifferent proportions of the elements within the compound, e.g., SiGeincludes Si_(x)Ge_(1-x) where x is less than or equal to 1, etc. Inaddition, other elements can be included in the compound and stillfunction in accordance with the present embodiments. The compounds withadditional elements will be referred to herein as alloys.

Reference in the specification to “one embodiment” or “an embodiment” ofthe present invention, as well as other variations thereof, means that aparticular feature, structure, characteristic, and so forth described inconnection with the embodiment is included in at least one embodiment ofthe present invention. Thus, the appearances of the phrase “in oneembodiment” or “in an embodiment”, as well any other variations,appearing in various places throughout the specification are notnecessarily all referring to the same embodiment.

It is to be appreciated that the use of any of the following “/”,“and/or”, and “at least one of”, for example, in the cases of “A/B”, “Aand/or B” and “at least one of A and B”, is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of both options (A andB). As a further example, in the cases of “A, B, and/or C” and “at leastone of A, B, and C”, such phrasing is intended to encompass theselection of the first listed option (A) only, or the selection of thesecond listed option (B) only, or the selection of the third listedoption (C) only, or the selection of the first and the second listedoptions (A and B) only, or the selection of the first and third listedoptions (A and C) only, or the selection of the second and third listedoptions (B and C) only, or the selection of all three options (A and Band C). This can be extended, as readily apparent by one of ordinaryskill in this and related arts, for as many items listed.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of exampleembodiments. As used herein, the singular forms “a,” “an” and “the” areintended to include the plural forms as well, unless the context clearlyindicates otherwise. It will be further understood that the terms“comprises,” “comprising,” “includes” and/or “including,” when usedherein, specify the presence of stated features, integers, steps,operations, elements and/or components, but do not preclude the presenceor addition of one or more other features, integers, steps, operations,elements, components and/or groups thereof.

Spatially relative terms, such as “beneath,” “below,” “lower,” “above,”“upper,” and the like, can be used herein for ease of description todescribe one element's or feature's relationship to another element(s)or feature(s) as illustrated in the FIGS. It will be understood that thespatially relative terms are intended to encompass differentorientations of the device in use or operation in addition to theorientation depicted in the FIGS. For example, if the device in theFIGS. is turned over, elements described as “below” or “beneath” otherelements or features would then be oriented “above” the other elementsor features. Thus, the term “below” can encompass both an orientation ofabove and below. The device can be otherwise oriented (rotated 90degrees or at other orientations), and the spatially relativedescriptors used herein can be interpreted accordingly. In addition, itwill also be understood that when a layer is referred to as being“between” two layers, it can be the only layer between the two layers,or one or more intervening layers can also be present.

It will be understood that, although the terms first, second, etc. canbe used herein to describe various elements, these elements should notbe limited by these terms. These terms are only used to distinguish oneelement from another element. Thus, a first element discussed belowcould be termed a second element without departing from the scope of thepresent concept.

Having described preferred embodiments of a method of device fabricationand a semiconductor device thereby fabricated to form a flexible siliconnanowire electrode (which are intended to be illustrative and notlimiting), it is noted that modifications and variations can be made bypersons skilled in the art in light of the above teachings. It istherefore to be understood that changes may be made in the particularembodiments described which are within the scope of the invention asoutlined by the appended claims. Having thus described aspects of theinvention, with the details and particularity required by the patentlaws, what is claimed and desired protected by Letters Patent is setforth in the appended claims.

What is claimed is:
 1. A method of forming a nanowire electrode, themethod comprising: forming a plurality of nanowires over a firstsubstrate; depositing a conducting layer over the plurality of nanowiresto form nanowire electrode arrays on the first substrate; formingelectrical interconnections over a flexible second substrate; formingsolder over the electrical interconnections; and integrating thenanowire electrode arrays of the first substrate to the flexible secondsubstrate by removing the first substrate from the nanowire electrodearrays and integrating the nanowire electrode arrays with the solderformed over the electrical interconnections on the flexible secondsubstrate.
 2. The method of claim 1, wherein the first substrate is asilicon (Si) substrate and the flexible second substrate is a flexiblepolymer substrate.
 3. The method of claim 1, wherein the plurality ofnanowires are silicon (Si) nanowires, the Si nanowires used as probes topenetrate at least a stratum basale layer of skin of a subject.
 4. Themethod of claim 1, wherein the conducting layer is a titanium nitride(TiN) layer.
 5. The method of claim 1, wherein the plurality ofnanowires are formed over the first substrate by metal-assisted chemicaletching.